The present invention relates to the forming of a Si3N4 thin film by atomic layer deposition (ALD). Specifically, the invention is embodied in the formation of a Si3N4 thin film by utilizing an atomic layer deposition method and employing Si2Cl6 (HCD) and NH3, or HCD and NH3 plasma as reactants.
Si3N4 thin films are becoming increasingly important in the manufacture of semiconductor devices. Si3N4 films at Si/SiO2 interfaces decrease interface traps and improve hot carrier immunity. Si3N4 films improve the reliability and performance of conventional SiO2 gate oxides. Si3N4 films at the SiO2/gate interface serve as excellent diffusion barriers, notably with respect to alkaline ions. In ultra thin film devices, Si3N4 could provide a high dielectric constant material that resists electron tunneling. Si3N4, having a higher dielectric constant than SiO2, could also prove very useful as a conformational insulating layer covering high aspect ratio features in DRAMS and other devices. These applications require a method of forming a Si3N4 thin film that exhibits good characteristics with respect to growth rates, thermal budget, pattern loading, purity, uniformity of thickness, and conformity to high aspect ratio features.
Deposition methods such as chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD) can be utilized for the preparation of a Si3N4 thin film. CVD-based methods often have drawbacks that limit their usefulness in the manufacture of semiconductor devices that would benefit by inclusion of thin films of Si3N4. In a typical CVD method, a thin film of SiN is deposited at a relatively high temperature, which in general is, less preferable than a lower temperature process due to the possibility of adverse thermal effects on the device. A SiN layer deposited by CVD is also subject to geometric hindrances causing thickness variations across the surface of the device. The thickness of the thin film formed around densely packed features on the surface can be less than the thickness of the film around less densely packed features. This problem is known as a pattern loading effect.
LPCVD suffers from shortcomings as well. The hydrogen content of the LPCVD-manufactured thin film is usually high, and step coverage of the surface is not good. Since the film growth rate is relatively slow when using LPCVD, the processing time required to grow a film of suitable thickness is relatively long. The long processing time exposes the substrate to a relatively high temperature for a long time, and results in a high thermal budget associated with the LPCVD process.
Atomic layer deposition (ALD) has been proposed as an alternative to CVD-based depositions methods for the formation of SiN thin films. ALD is a surface controlled process conducted in a surface kinetic regime, and which results in two-dimensional layer-by-layer deposition on the surface. Goto et al. describe an ALD deposition method using dichlorosilane (DCS) and NH3 plasma to form a Si3N4 film. (Appl. Surf. Sci., 112, 75-81 (1997); Appl. Phys. Lett. 68 (23), 3257-9 (1996)). However, the properties of the thin film manufactured by the method described in Goto are not suitable. The Cl content (0.5%), and O content are unacceptably high. These, combined with a measured Si:N ratio of 41:37 indicate that this method does not form a near-stoichiometric film of Si3N4. In addition, the growth rate of 0.91 angstroms per cycle of 300 seconds is not as high as would be necessary for commercial applications.
Klaus et al. describe an ALD method of forming a Si3N4 film by reacting SiCl4 and NH3. See, U.S. Pat. No. 6,090,442, and Surf. Sci., 418, L14-L19 (1998). The characteristics of the thin film manufactured by this method are better than that achieved by Goto et al. The ratio of Si:N=1:1.39, and the Cl, H and O contents are acceptably low. However, the cycle time of 10 minutes to grow a 2.45-angstrom film is too long, making any commercial application impractical.
It has also been proposed to use Si2Cl6 (HCD) and N2H4 to deposit a thin Si3N4 film by ALD. (Appl. Surf. Sci., 112, 198-203 (1997)). While the stoichiometry, Cl and H content of such films are suitable, they exhibit an unacceptably high oxygen content, rendering such films unsuitable for the uses described above.
Therefore, despite a long-recognized potential for widespread application, a need remains for a novel method of forming Si3N4 films that meet the following criteria: low thermal budget process; excellent step coverage; no pattern loading effect; Si:N ratio consistent with Si3N4; excellent thickness control and uniformity; minimal number of particulate inclusions; low impurity content; and a film growth rate that makes commercial application practical.
It is therefore an object of this invention to provide a method of forming Si3N4 as a film in which the Si3N4 embodies physical and chemical properties consistent with highly pure Si3N4. It is another object of the invention to provide a method of depositing Si3N4 as a thin film in which the method demonstrates excellent step coverage, little or no pattern loading effect, and excellent thickness control and uniformity. It is a further object of the invention to provide a method of depositing Si3N4 as a thin film or other solid form wherein the method demonstrates a relatively low thermal budget and an acceptably high growth rate to render the method practical for commercial application.
In order to accomplish the above-described items, the present invention is embodied in an atomic layer deposition (ALD) method employing Si2Cl6 and NH3, or Si2Cl6 and activated NH3 as reactants.
In one embodiment, the invention includes the steps of a) placing a substrate into a chamber, b) injecting a first reactant containing Si2Cl6 into the chamber, c) chemisorbing a first portion of the first reactant onto the substrate and physisorbing a second portion of the first reactant onto the substrate, d) removing the non-chemically absorbed portion of the first reactant from the chamber, e) injecting a second reactant including NH3 into the chamber, f) chemically reacting a first portion of the second reactant with the chemisorbed first portion of the first reactant to form a silicon-containing solid on the substrate, and g) removing the unreacted portion of the second reactant from the chamber. In another embodiment of the invention, in step b, the first reactant contains two or more compounds, each containing Si and Cl. In a preferred embodiment thereof, the two Si and Cl containing compounds are Si2Cl6 and SiCl4. In another embodiment of the invention, steps b-g are repeated one or more times to increase the thickness of the layer.
In yet another embodiment of the invention, the method includes the steps a) placing a substrate into a chamber, b) injecting a flow of a first reactant containing Si2Cl6 into the chamber, c) while injecting the first reactant into the chamber, adding SiCl4 to the flow of the first reactant, d) chemisorbing a first portion of the first reactant onto the substrate and physisorbing a second portion of the first reactant onto the substrate, e) chemisorbing a first portion of the SiCl4 onto the substrate and physisorbing a second portion of the SiCl4 onto the substrate, f) removing the non-chemically absorbed portions of Si2Cl6 and SiCl4 from the chamber, g) injecting a second reactant including NH3 into the chamber, h) chemically reacting a first portion of the second reactant with the chemisorbed first portion of the Si2Cl6 and the chemisorbed first portion of SiCl4 to form a silicon-containing solid on the substrate; and, i) removing a second portion of the second reactant from the chamber. In another embodiment, one or more of steps b-i are repeated. These and other features of the invention will now be explained in greater detail by reference to the following drawings and detailed description.